Traction control circuits have been incorporated into anti-lock brake system to provide a way of synchronizing the rotational spin of the wheels of a vehicle. U.S. Pat. Nos. 4,778,225, 5,096,267 and 5,176,433 illustrate structure to accomplish such traction control in a manner acceptable to most vehicle operators. Unfortunately, during the activation of the traction control, the brake pedal which is directly connected to the movable wall of the vacuum brake booster moves during the activation of the traction control circuits or in order to incorporate the traction control structure into the brake system, the vacuum brake booster must be modified through the addition of various internal and external conduits which can significantly increase the cost of a brake system.
In co-pending patent application No. 08/068,881 filed May 28, 1993, traction control for a brake system was developed by modifying a standard production brake booster through the addition of a traction servomotor to the input member. The input member was designed to be responsive to either an input force applied through an operator pedal assembly or an input force produced by a traction servomotor responding to a signal derived from a difference in rotational spin of the drive wheels to effect a brake application of the wheels in a vehicle. The housing for the traction servomotor which was attached to the movable wall of the brake booster retained a movable wall which separated a first chamber from a second chamber. In response to an input signal derived by a controller, a pressure differential was developed across the movable wall to produce a traction control output force. This traction control output force was applied to the input member to move the control valve and selectively effect a brake application and synchronize the rotation spin of the wheels. This structure operates in a satisfactory manner, however the size of the traction servomotor required to develop an output force to operate the control valve added considerable weight to the overall brake system.